This invention relates to the recovery of oil from subterranean oil reservoirs and more particularly to improved waterflooding operations involving the injection of thickened aqueous solutions of copolymers of amphoteric polyelectrolytes for mobility control.
In the recovery of oil from oil-bearing reservoirs, it usually is possible to recover only minor portions of the original oil in place by the so-called primary recovery methods which utilize only the natural forces present in the reservoir. Thus a variety of supplemental recovery techniques have been employed in order to increase the recovery of oil from subterranean reservoirs. The most widely used supplemental recovery technique is waterflooding which involves the injection of water into an oil-bearing reservoir. As the water moves through the reservoir, it acts to displace oil therein to a production system composed of one or more wells through which the oil is recovered.
One difficulty often encountered in waterflooding operations is the relatively poor sweep efficiency of the aqueous displacing medium; that is, the injected displacing medium tends to channel through certain portions of the reservoir as it travels from the injection system to the production system and to bypass other portions. Such poor sweep efficiency or macroscopic displacement efficiency may be due to a number of factors such as differences in the mobilities of the injected displacing liquids and the displaced reservoir oil and permeability variations within the reservoir which encourage preferential flow through some portions of the reservoir at the expense of other portions.
Various techniques have been proposed in order to improve the sweep efficiency of the injected displacing medium and thus avoid premature breakthrough at one or more of the wells comprising the production system. The most widely used procedure involves the addition of thickening agents to the injected displacing medium in order to increase the viscosity thereof and thus decrease its mobility to a value equal to or less than the mobility of the displaced reservoir oil, resulting in a "mobility ratio" of oil to water which is less than or equal to one. Many polymeric thickening agents including both anionic and cationic polyelectrolytes have been proposed for use in such mobility control operations. Thus, U.S. Pat. No. 3,085,063 to Turbak discloses waterflooding in which the water is thickened by the addition of polyvinyl aromatic sulfonates such as sulfonated polystyrene and copolymers of such vinyl aromatic sulfonates. Similarly, U.S. Pat. No. 3,984,333 to van de Kraats et al. discloses waterflooding involving the injection of an aqueous solution thickened by block copolymers in which the water-soluble blocks are sulfonated polyvinylarenes and the relatively water-insoluble blocks are polymerized alpha olefins and/or hydrogenated dienes such as polyisoprene and polybutadiene. Synthetic anionic polymers such as those disclosed in Turbak and Kraats et al., as well as the more widely used partially hydrolyzed polyacrylamides, suffer a number of disadvantages in actual operations. Where the injected water or the reservoir water contains significant quantities of dissolved inorganic salts, their viscosity yield is decreased materially. In addition, the presence of divalent cations such as magnesium and calcium tend to cause precipitation of these anionic polymers. Biopolymers such as the Xanthomonas polysaccharides retain much of their thickening power in the presence of inorganic salts and thus may be employed in high brine evironments. However, in the absence of special stabilizing procedures, these polysaccharides are subject to severe thermal-hydrolytic degradation at temperatures of about 60.degree. C. and above which limits their application in relatively high temperature reservoirs.
It also has been proposed to employ cationic polyelectrolytes as thickeners in waterflood applications. Thus, U.S. Pat. No. 3,744,566 to Szabo et al. discloses the use of a water-soluble polymer containing at least 1% cationic monomer units characterized as acrylamido quaternary ammonium halides, sulfonates, carboxylates, etc. The cationic monomer may be copolymerized with other copolymerizable water-soluble monomers such as acrylamide, alkali metal styrene sulfonates, and N-vinylpyridine. The polymers disclosed in Szabo et al. are said to be particularly useful in brines having over 2% dissolved solids.